With age, circulating testosterone levels decline in males, resulting in conditions ranging from decreased sexual function to cognitive decline, metabolic disorders, and increased cardiovascular disease risk. As a result, aged males, and in some cases young males with clinically low testosterone concentrations, are prescribed long-term testosterone replacement therapy. In preliminary studies, we found that the immune system also is a target of testosterone, affecting the outcome of infectious diseases, including influenza. Using a murine model, we show that low testosterone concentrations, either endogenously occurring in aged male or caused by surgical castration (i.e., hypogonadism) in young males, increase inflammation and result in greater morbidity and mortality following influenza virus infection as compared with young males that have high circulating concentrations of testosterone. In this R21 application, we propose to critically test the hypothesis that low circulating testosterone concentrations contribute to age-associated changes in susceptibility to influenza and in vaccine efficacy by altering immune responses in males. We further predict that testosterone replacement in either young or aged males with low testosterone levels will protect against influenza infection and improve the outcome of vaccination. In Aim 1, we will determine whether age-dependent reduced testosterone in males affects the kinetics and magnitude of virus replication as well as inflammatory, cell-mediated, and humeral immune responses to influenza infection. We will also establish whether treatment with testosterone protects against disease, particularly in older males. Vaccination against influenza is the most effective means for reducing influenza, yet the efficacy of vaccines declines with age, especially in males. Aim 2 will test the hypothesis that low testosterone levels reduce antibody responses to influenza vaccination and protection against lethal challenge with influenza viruses in males. Replacement of testosterone in either young or aged males with low testosterone levels should improve vaccine efficacy by directly altering humeral immune responses. During seasonal epidemics of influenza as well as during the recent H7N9 outbreak, a significant majority of the deaths occur in people over 65 years of age, with men being 2-times more likely to die than women. If testosterone replacement therapy provides significant protection for males against influenza, then this could provide an inexpensive supplemental therapeutic approach for saving lives and reducing disease burden during influenza epidemics and pandemics.